Communications systems are well known. Conventional electronic communications rely on electrons passing through wires. Radio-frequency and microwave communications rely on radio waves and microwaves travelling through open space. The crucial difference between these systems and fiber optic communication systems is that, in the latter, signals are transmitted as light.
The advantages of fiber optic communications have led to many applications in long-haul and short-distance communications. Examples include optical local-area networks, under-sea telecommunication systems, connections between control facilities, cable television, and portable communication equipment for military use.
A key element of a fiber optic communication system is the optical fiber. The two key elements of an optical fiber are its core and cladding. The core is the central region of the optical fiber, through which light is guided. The cladding is the outer region, completely surrounding the core. Light traveling through the core is confined in the core. Light that strikes the core/cladding boundary remains in the core because the refractive index of the core is higher than that of the cladding.
A splitter is an optical fiber coupling device having at least one input optical fiber and at least one output optical fibers. Functionally, a splitter optically connects the input optical fiber to the output optical fibers so that light energy is divided among the output optical fibers. The input light energy need not be divided equally among the output optical fibers. For example, it may be desirable for approximately 10% of the input light to emerge from one output and approximately 90% from another. Many applications require branching a video signal for cable television to several streets in a neighborhood. In such a system, the splitter must take the incoming signal from the main office and split the signal between the streets and the subsequent homes on the streets. A splitter will be used at each split point.
A combiner is an optical fiber coupling device having at least two input optical fibers and at last one output optical fiber. Functionally, a combiner optically connects the input optical fibers to the output optical fibers so that light energy is combined into the single output optical fiber.
A coupler transfers light energy between two coupled optical fibers. A coupler may be utilized as either a splitter or combiner.
Manufacturers of optical fiber couplers usually provide devices which are designed to operate at preselected operating wavelengths or have fixed coupling characteristics. Customers, on the other hand, often require devices that have variable coupling characteristics and operating wavelengths or allow changing the spectral properties after installation.
Many couplers operate on the principle that coupling occurs between the cores of two adjacent optical fibers. Early couplers were made by applying heat directly to the optical fibers to be joined, producing a very fragile coupler. Because of their small size, these fused couplers were inherently less robust and environmentally unstable. To provide a mechanically strong coupler, the optical fibers may be joined together in a matrix glass, referred to as an overclad, having a refractive index less than that of the cladding material and tapered to bring the optical fiber cores closer together to induce coupling.
Overclad fiber optic couplers are conventionally formed, as illustrated in FIGS. 1-4, from a glass capillary tube 13. As shown in FIGS. 1 and 2, tube 13 has a longitudinal aperture 16 which may be diamond shaped, as depicted in FIG. 2. Two waveguides 11,12 are positioned within diamond shaped aperture 16, as shown in FIG. 2, extending through tube 13. The midregion of tube 13 is heated, collapsed about waveguides 11, 12 and drawn to reduce the diameter. As a result, and as shown in FIG. 3, a tapered mid-region 14 is formed. As shown in FIG. 4, the cores 11a and 12a are positioned adjacent each other in the midregion 14.
As a result, light input to waveguide 11 is distributed between the output of waveguide 11 and/or waveguide 12 of FIG. 3, as determined by the coupling ratio. For example, light of a single wavelength initially enters waveguide 11 but not waveguide 12. Light traveling along waveguide 11 is transferred from waveguide 11 to waveguide 12. The amount of light transferred is dependent upon the geometry of coupling region 14, cores 11a, 12a and claddings 11b, 12b and their refractive indices, and the refractive index of the overcladding tube 13.
Alternatively, light of a single wavelength may initially enter waveguide 12 but not waveguide 11, wherein the light traveling along waveguide 12 is transferred from waveguide 12 to waveguide 11. The input and output pigtails of waveguides 11, 12 may be kept intact, so that in the event one input becomes inoperable the coupler will remain operable.
When substantially all the input light is to be distributed first to the output of one optical fiber and alternatively to the output of a second optical fiber, a switch is required. An optical switch merely transfers substantially all input light of a single wavelength from one output optical fiber a second output optical fiber. A switch is a two state device, operating either on or off. The coupler described above be used as a switch, wherein the output of the coupler may be controlled by bending the coupler. The fiber-in-tube coupler of FIG. 3 is stiff enough to allow for reproducible and easily controlled bending, yet it is not so large as to require large forces to bend the coupler.
Some applications require a coupler wherein one can vary the division of the input signal between the first and second output optical fibers. The term for this device is a variable tap. A variable tap operates in many states; the coupling ratio is not limited to two states, as in the optical switch. In one form of a variable tap, the angle of bending of the coupler is varied whereby the light is selectively transferred among the output optical fibers.
Conventional fiber optic switches typically consist of optical fibers with opposing end sections capable of connection or disconnection by various means. A prior art fiber optic switching device for use in optical fiber systems is disclosed in J. Lemonde U.S. Pat. No. 4,759,597, issued on Jul. 26, 1988. The disclosed switch consists of an optical fiber with a moving end which can be displaced between two positions. The moving end is mechanically aligned with the fixed end of a second or third optical fiber. The transmission path is switched from the first optical fiber to the second or third optical fiber. A significant problem with this design is the fact that the necessary accuracy required for alignment of the optical fibers is difficult to achieve. Another drawback is the difficulty of obtaining a low attenuation coefficient due to the gap between the optical fibers.
S. Battle U.S. Pat. No. 4,753,501, issued on Jun. 28, 1988, discloses an optical switch also utilizing alignment of optical fibers to switch the transmission path. The input optical fiber is connected to a rotary shutter. The input optical fiber is rotated to a number of different positions for alignment with output optical fibers. This switch suffers from the same drawbacks as the Lemonde switch, alignment inaccuracy and high attenuation.
H. Lee U.S. Pat. No. 4,896,935, issued on Jan. 30, 1990, discloses a fiber optic switching device comprising input and output optical fibers to be optically aligned for switching the optical paths. The fixed input optical fibers may be aligned with moveable output optical fibers. The output optical fibers are moved between input optical fiber positions. An alternative embodiment is directed to rotation of an array of output optical fibers for alignment with the input optical fibers.
The problem of aligning optical fibers for switching was recognized in the art. One of the approaches to solving this problem, which is evidenced by the prior art, is the concept of perturbation, for example bending the tapered region of a pair of waveguides, thereby altering its coupling performance.
In one type of such a device, a two refractive index structure is bent in order to switch the transmission paths. For example, EP 0 048 855 A3 by E. Klement discloses a dual core optical fiber which is bent to change the degree of coupling between the cores. When the optical fiber is bent, the first core is compressed and the second core is elongated. Reverse bending of the dual core optical fiber suspends the coupling of the cores.
B. Kawasaki et al. U.S. Pat. No. 4,763,977, issued Aug. 16, 1988, is also directed to a coupler comprising a pair of optical fibers fused together at a narrowed region. The coupler is bent in the narrowed region, whereby a coupling ratio can be selected.
Although the Klement and Kawasaki bending devices avoid the alignment and low attenuation drawbacks, the couplers are sensitive to vibration, pressure, temperature, etc. and therefore reproducibility of the coupling ratio may vary over time. Also, a substantial force is required to bend the coupler each time the switch is used.
It is therefore an object of the present invention to provide a variable optical tap that requires less force to vary the coupling ratio.
Another object of the present invention is to provide a variable optical tap that is less sensitive to perturbation.
Another object of the present invention is to provide an optical switch wherein the transmission signal never leaves the glass structure, eliminating the air to glass interface.
Another object is to provide a thermally stable variable optical coupler.